As used herein, the term “functional material” refers to an anisotropic material having electrical, optical, thermal, chemical or mechanical properties which change in the horizontal, vertical or diagonal direction with respect to the surface, a material having physical properties which differ according to height, or a material having physical properties which change regularly, irregularly or continuously according to height in a graduated manner. That is, it refers to a material showing various functions because of such anisotropy or physical properties which change in a graduated manner.
Functional materials having such physical properties may be used as thermoelectric materials in kitchen utensils, electric home appliances or thermoelectric elements, and such anisotropy or physical properties changing in a graduated manner also have an application in various industrial fields.
Among materials having such physical properties, thermoelectric materials with important electrical and thermal properties will now be described.
Thermoelectric materials are used in thermoelectric elements for thermoelectric generation and thermoelectric cooling, and typical thermoelectric materials are metallic thermoelectric materials represented by Bi. The most often used metallic thermoelectric materials include Bi—Ag, Cu-constantan, Bi—Bi/Sn alloy, BiTe/BiSbTe, etc. Recently, semiconductor thermopiles having Seebeck coefficients higher that the metal-based materials have been most often used; however, in fields requiring stability, metallic thermopiles are most frequently used. Metallic thermopiles have an advantage of low noise due to low resistivity. However, they have low sensitivity due to a low Seebeck coefficient. For example, in Cu having a Seebeck coefficient of almost zero, the electromotive force caused by the temperature difference does not occur. Among metallic materials, Bi is used as a thermoelectric material due to low thermal conductivity and a high Seebeck coefficient.
In comparison with such metallic thermoelectric materials, semiconductor thermoelectric materials represented by Si show excellent sensitivity due to having a high Seebeck coefficient and, in addition, may be applied directly to existing IC processes. Due to such advantages, the semiconductor thermoelectric materials are being very widely used.
Generally, the thermoelectric performance of the thermoelectric materials is determined by physical properties including thermoelectromotive force (V), Seebeck coefficient (α), Peltier coefficient (π), Thomson coefficient (τ), Nernst coefficient (Q), Ettingshausen coefficient (P), electrical conductivity (σ), powder factor (PF), figure of merit (Z), dimensionless figure of merit (ZT=α 2 σT/K wherein T is absolute temperature), thermal conductivity (κ), Lorentz ratio (L), electric resistivity (ρ), etc.
Particularly, the dimensionless figure of merit (ZT) is an important factor in the determination of thermoelectric conversion efficiency, and when a thermoelectric element is manufactured using a thermoelectric material having a high figure of merit (Z=α 2 σ/K), it can have an increased efficiency of cooling and powder generation.
Accordingly, it is particularly preferable to use a thermoelectric material having a high Seebeck coefficient (α) and electrical conductivity, and thus a high power factor (PF=α 2 σ). It is most preferable to use a thermoelectric material having a low thermal conductivity (κ) in addition to such preferred properties. Moreover, it is preferable to use a thermoelectric material having a high Seebeck coefficient (α) together with a high ratio of electrical conductivity to thermal conductivity, σ/κ (=1/TL; mainly in the case of metals).
Various attempts have been made to increase the thermoelectric performance of thermoelectric materials, but such methods have been limited mainly to changing the composition ratio or the kind of elements composing the thermoelectric material.
Such thermoelectric materials are manufactured by powdering the components of the thermoelectric material, sintering and molding the powder, and cutting the molded material for use. In another attempt to improve thermoelectric performance, there is a method of forming a stack structure using MBE (molecular beam epitaxy) or CVD (chemical vapor deposition), thus increasing the dimensionless figure of merit (ZT). However, these methods have problems in that they do not sufficiently improve thermoelectric performance and, in addition, are not economically viable due to a long production time being required.